ViewSonic VA2016W - 20" LCD Monitor Service Manual page 34

R823, R824 and R822. R823 R824 and R822 are output sampling resistor. When the
sampling voltage more than 2.5V or less than 2.5V, feedback current of IC802 will change,
this can change the voltage from transformer T801.
For output OVP, ZD803,ZD804 and ZD805 are zener diode, when the voltage add to the
zener up to their's rating voltage 5.6V, 3.9V or 20V, the zener's leakage current cause R828
voltage become up to 0.7V, Q803 is triggered and OVP starts. The collector current of Q803
is used to make build-in diode light. FB Current of IC802 will be changed; it can change the
output voltage from T801.
2. Inverter Circuit
2.1 Low voltage to high voltage circuit
14VDC provides the power for IC501; the control signals Brightness and ON/OFF come
from I/F board. ON/OFF signal connect to pin8 of IC501 and makes IC501 enable. Brightness
signal connect to pin7 of IC501 and regulates the panel brightness, R524, R529, C505 make up
a network of delaying time circuit and R528, R523, R524 make up a divided voltage network,
C504 is used to dump noise. The operation frequency is determined by the external Resistor
R522 and capacitor C529 connected to pin5 of IC501. BURST MODE regulated dimming
frequency is determined by the external resistor R527 and capacitor C506 connected to pin6 of
IC501. C502 is used for soft start and compensation, C502, C528 are used for dump noise.
The output drives, include NDR4, NDRV2, PDRV3, PDRV1 (pins1, 3, 15, 16 respectively)
output square pulses to drive MOSFET U501, U502, and each of U501, U502 is consist of a N
channel MOSFET and a P channel MOSFET. U501 and U502 work as full-bridge topology, it is
high efficient, zero voltage switching.
During start up, VSEN (pin9) senses the voltage at the transformer secondary. When VSEN
reaches 3.0V, the output voltage is regulated. If no current is sensed approximately 1.5 seconds
IC501 shunt off.
The current flowing through CCFL is sensed and regulated through sense resistor R509,
R511. The feedback voltage through R506, R507, C508 connected to Pin11 (ISEN), then
compared with a reference voltage (1.5V) via a current amplifier, resulting in PWM drive outputs
to full-bridge switches.
2.2 Protection circuit
Over Voltage Protection: R501and R502 are connected in high voltage output connector, the
divided AC voltage is inverted DC voltage through D508, R505 and C507are used to rectify wave
& dump noise. Then the voltage signal reaches Pin9 VSEN of IC501, when the voltage changes,
build-in PWM of IC501 will adjust output voltage.
Open Lamp Protection: In normal operation, the resistors R510, R511, R512, R509 are
sensed a high level AC voltage, the AC signal IS1 invert DC voltage through D509, R515, C533,
and the high level DC voltage reaches the gate pin of Q502, similarly, the gate pin of Q503, Q504,
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